Bleomycin is an important anticancer antibiotic whose primary biochemical action is to cleave chromosomal DNA generating both single and double strand scission. The absence of immunosuppression by bleomycin at therapeutically relevant concentrations has lead to its increased importance for the treatment of tumors in immunosuppressed patients, e.g., AIDS. Double strand scission of DNA mediated by bleomycin occurs in excess of that expected from the random accumulation of single strand nicks and represents a lesion that is refractory to repair in eukaryotes. A new model is proposed that double strand scission by bleomycin is a form of self-potentiation whereby sites that are initially damaged by bleomycin are preferentially targeted for further damage. The proposed research represents an integrated approach to address the structural and chemical basis of bleomycin-mediated damage of DNA. The chemistry of double strand scission will be characterized and the structure/activity relations of target oligonucleotides and synthetic models of damaged DNA will be investigated. The interactions responsible for the sequence specificity of bleomycin damage of DNA and double strand scission will be investigated using two dimensional NMR experimentally. The long term objectives are to understand the principles that govern the mechanism of bleomycin-mediated double strand scission and the related questions of the molecular basis for the sequence-specific targeting of bleomycin. The results can provide a rational basis for the formulation of new therapies involving bleomycin in combination with other drugs and in the design of new chemotherapeutic agents that conduct self-potentiating damage of nucleic acids. Finally, the models for nicked DNA can be used to assay for drug activity targeted against damaged DNA.